Engineering Geology on the Bypass EarthCache

This cache has been archived.

spooky_luke: Many thanks to all who have visited over the years, but the vegetation has now partially obscured the engineering geology features that this EarthCache was designed to showcase, so I think it is time for it to go.

More
Hidden : 9/1/2016
Difficulty:
2.5 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

Join now to view geocache location details. It's free!

Watch

How Geocaching Works
Looking for a different adventure?

Please note Use of geocaching.com services is subject to the terms and conditions in our disclaimer.

Geocache Description:

This EarthCache is designed to highlight a variety of geotechnical aspects relating to ground stability. It is located along a public footway on a newly-created section of the A391 through an area that has previously been used for China Clay extraction.

What is Engineering Geology?

Engineering Geologists use their skills, knowledge and understanding of geological sciences and engineering to ensure that the geological factors regarding the location, design, construction, operation and maintenance of any engineering works are properly dealt with.

Put simply, they analyse ground conditions and then design engineering solutions to ensure that the structures they build are not only cost-effective, but safe. The range of construction projects that Engineering Geologists work on include (but is not limited to) roads, railways, bridges, tunnels, dams, seawalls, mines and quarries.

History of Engineering Geology

The hard-learnt lessons from constructing embankments, cuttings, tunnels and bridges for railways, canals and other major engineering projects in Victorian Britain needed to be shared. The first book published on Engineering Geology was in 1880 by British Engineer and Geologist William Henry Penning.

William Henry Penning


However, the person who arguably made the biggest contribution to the development of Engineering Geology as a Science was the Austrian Engineer and Geologist Karl von Terzaghi.

Karl von Terzaghi

Widely known as the ‘Father of Soil Mechanics’, Terzaghi devised testing equipment and protocols to actually measure the strength of different ground conditions. The data that he was able to obtain then enabled him to engineer and install solutions to ensure that structures were stable and safe.

Geotechnical Ground Engineering Solutions

A variety of different methods are used by Engineering Geologists, depending on what the ground situation dictates. This can include the following:

Rock Bolting and Soil Nailing
Where rock and/or soil is unstable and liable to failure (collapse or undesirable movement), rock bolts and/or soil nails can be used to permanently stabilise the ground by ‘pinning’ it together and spreading the forces acting on the unstable face.
Rock bolts were first used in underground mining in the 1890s and then for tunnelling in the US and in Australia in the late 1940s.
Some rock bolts comprise solid rebar (reinforcing bar) steel rods that are inserted into pre-drilled holes and fixed into place using a type of cement or grout. The grout is often a 2-part resin with a setting time of anything between 15 seconds and 7 minutes. The greater the amount of unstable rock present, the longer the rock bolts and more closely-spaced they are installed.

Steel Rebar Rock Bolt

Other rock bolts can be expanded to anchor them into place once inserted into the hole, by either tightening the nut on the end, or by pumping high-pressure water into a hollow section of the bar.

Soil nails are similar to rock bolts and were first used in 1972 to facilitate widening a railway near Versailles in France. Like rock bolts, soil nails can be either solid or hollow and are driven into an area of sloping unstable ground and grouted into place. To save time, reduce effort and therefore cost, some soil nails are installed and grouted simultaneously. These have a sacrificial drill head on the end with holes in to allow the drill hole to be flushed and the grout to be injected.

Hollow Soil Nail with Sacrificial Drill Head

You can identify where a soil nail of this type has been used as the part of the nail that protrudes from the hole is hollow and there is often excess grout spilt on the face below the level of the hole.
If the material into which a rock bolt or soil nail is inserted is especially unstable, additional welded or chain-link mesh and geotextile membrane can also be used to hold-back the smaller looser parts of the exposed face as shown in the picture below:

Unstable Face showing Soil Nails, Geotextile Membrane and Chain-link Mesh

Shotcrete can be used (often in conjunction with rock bolts and welded mesh) to rapidly cover and seal a particularly loose rock face to prevent any rock falls. Invented in 1907 by American taxidermist Carl Akeley to repair the crumbling facade of the Field Columbian Museum in Chicago, pre-mixed concrete is sprayed from a high-velocity nozzle onto the rock face. This particular method is commonly used in the construction of tunnels where a fast-acting but permanent stabilisation technique is required.

Shotcrete and Welded Mesh

Retaining Walls
Instead of bolting or nailing a rock face, a wall can be constructed to retain unstable material whilst keeping a near-vertical face and also providing a more durable or aesthetically pleasing surface. Several different types of retaining wall exist as shown below:

Types of Retaining Wall

Gravity walls are often constructed using local stone to give a more attractive finish than concrete, or fully vegetated to hide the wall altogether. A variation on this is the use of gabions – mesh cages filled with heavy stone then stacked and wired together as shown below:

Gabion Wall

Piling walls can be very strong, especially if interlocking steel piles are used. This is particularly useful for riverside, dock and harbour walls as shown below:

Anchored Piling Wall

Cantilever walls are very stable and can be prefabricated off-site, lifted into place and joined together as shown below:

Pre-Fabricated Cantilever Wall Sections

Alternatively, cantilever walls can be formed in-situ using a specialist machine. Many modern central reservation crash barriers use a cantilever wall as it was found that they prevented larger 4x4-type vehicles and lorries from flipping and crossing over the carriageway (as shown below) whereas traditional steel cables or barriers could not.

Cantilever Barrier Strength Test

Any type of retaining wall can be anchored using cables, bolts or stays anchored into the rock or soil behind the wall. Anchoring is necessary where a road cutting is very deep, high loads are expected, or where the wall itself has to be very thin and/or very high so would otherwise be too weak to support the unstable ground behind it. An example of an anchored wall is shown below:

Anchored Retaining Wall


Angle of repose
Another less sophisticated but equally valid way to create a stable slope is to grade (shape) the slope in such a way so as to not exceed the angle of repose. Defined as the steepest angle of descent or dip relative to the horizontal plane to which a (granular) material can be piled without slumping, different materials can be graded to different angles before they become unstable.

Angle of Repose - The angle between the horizontal and the sloping face


If the amount of land area available for an engineering project is unlimited, any sloping ground involved in the project could just be graded to less than the angle of repose thereby negating the need for any of the aforementioned engineering solutions. However, as the land available for engineering projects is often limited, at least some of the slopes have to be cut to an angle greater than the angle of repose thereby requiring additional geotechnical interventions.

Close to the headline coordinates lies the unmistakable Great Treverbyn Sky Tip. Composed primarily of waste sand from the nearby Great Treverbyn China clay pit, construction of the tip started in 1914 and finished in 1958. The sand was tipped using a conveyor system and the slope of the tip formed naturally as the material slumped and finally settled to approximately the angle of repose as shown below:

Conveyor System being used to create two Sky Tips in the 1950s

However, after the Aberfan disaster in 1966, building these steep-sided spoil tips was discontinued in favour of wider, flatter terraced spoil tips as shown below:

Modern Terraced Spoil Tip

In order to claim this EarthCache, go to the published coordinates - You should be near a bench with some obvious Engineering Geology visible on both sides of the road. N.B., For safety, it is not necessary to cross over the road to look more closely at the rocks on the other side.

Please message or email me your answers to the following questions through my profile at the top of this page or via this link:-

1) Observe the rock that is exposed here. Describe what it looks like and whether or not you think it is stable.

2) Explain what (if any) geotechnical schemes are present at GZ. HINT Use the information in the description to help you identify if any of the schemes mentioned have been used here.

3) Walk under the bridge and along the path towards Additional Waypoint 1. On your way, notice that the embankment to your right is stable as it has been graded to be less than the angle of repose. However, at Additional Waypoint STAGE 1, the slope needed to be steeper than the angle of repose. Suggest how you think the Engineers have ensured that the embankment is still stable at this location.

4) Continue along the path (back towards the parking spot), then take the path to the right up towards the footbridge to reach Additional Waypoint STAGE2. Once there, you are standing at the foot of the Great Treverbyn Sky Tip. Estimate the Angle of Repose for this tip and describe any evidence you can see to suggest whether or not this huge tip is stable.

 Feel free to log your find without having to wait for confirmation from me. However, if your answers are not correct or incomplete etc., I will contact you to verify your find.

Earthcache guidelines now make posting a photograph optional, but feel free to upload pics with your log if you wish.

Thanks for taking the time to visit this Earthcache and I hope you enjoy your visit.

Additional Hints (No hints available.)



Advertising with Us